1. Field
Example embodiments relate to image sensors having a ramp generator, for example, a ramp generator and method for calibrating a ramp slope value of a ramp signal.
2. Description of the Related Art
Conventional complementary metal-oxide semiconductor (CMOS) image sensors are commonly used in portable cameras, digital still cameras, and web cameras, for example, to convert an image into a digital signal. The conventional CMOS image sensor uses a correlated double sampling (CDS) method and a ramp signal to generate a digital signal. For example, the CMOS image sensor determines the difference between a reset signal and an image signal that varies based on external light illumination. The CMOS image sensor may generate a digital code corresponding to the difference between the reset signal and the image signal. The digital code may vary depending on the slope of the ramp signal. To maintain the same luminosity or brightness for different samples or different operating conditions of the same sample (including intentional clock rate change, for example), the ramp signal must be constant when the image is acquired by the CMOS image sensor.
Conventionally, an analog ramp calibration scheme may generate and calibrate a constant ramp signal. In the conventional analog scheme, the ramp signal may be adaptively calibrated to coincide with an analog target voltage using an analog comparator. The accuracy of the ramp calibration process may affect the accuracy of the ramp signal, which in turn, may affect image quality.